1. Field of the Invention
The present invention relates to data writing (recording) on a magnetic medium. More particularly, it is applicable to the high surface density writing of data on the magnetic disks of magneto-optical storages used mainly in data processing systems.
2. Description of the Prior Art
In data processing systems, the result of the operations performed by a central processor unit (CPU) is either utilized and analyzed immediately by the user or stored for variable periods of time in stores such as mass memories or bulk storages. Magnetic disks are the most frequently employed as the storage element. The data carried by the magnetic disks is stored within circular concentric recording tracks having a radial width of the order of a dozen microns or more and usually covers the major part of the two sides of the disks.
As a rule, a string of magnetic data written on a track of a disk is present in the form of a succession of small magnetic domains called "elementary domains" distributed throughout the length of the track and having magnetic inductions with the same modulus and opposite direction.
Line density is defined as the number of changes in the direction of magnetization per unit length measured in accordance with the circumference of a track, and radial density is the number of tracks per unit length measured in accordance with the diameter of the disk. Surface density is the number of data per unit of surface. In high density stores, one aims at obtaining radial and per unit length densities of the order of, respectively, 5000 tracks per cm and 10,000 changes in the direction of magnetization per cm.
The means by which data can either be written (or recorded) or read on the disks are called transducers. As a rule, one side of a given disk is associated with one or more transducers, with the disk moving past the transducer(s).
In the development of magnetic disk stores, one of the current developments is magneto-optical stores where the data is written on magnetic disks, while the reading is effected by a set of opto-electronic devices. Opto-electronic reads permit one to observe at a given moment and at a given location one side of a disk by means of a polarized light beam and to deliver an electric signal whose voltage or current is dependent upon the value of the data present at that location. This signal is supplied by at least one photoelectric transducer which receives the polarized light from the light whose properties are modified on contact of the disk's side so that, for example, if the beam comes into contact with a magnetic domain of negative magnetization, the photoelectric transducer delivers a signal with a non-zero voltage.
The mode or method of writing (or recording) data on the magnetic disks of magneto-optical storages is such that the magnetization in the elementary domains is perpendicular to the magnetic recording layer of the disk. This type of magnetization permits one to obtain greater longitudinal and radial densities of data and its mode of observation by means of a light beam is easier than the mode of observation of an environment where the magnetization is longitudinal, that is to say, parallel to the surface of the layer. This mode of recording data is called a "perpendicular recording mode". In this case, the magnetic environment constituting the layer is a magnetic anisotropic environment, that is to say, an environment having one or more privileged directions of magnetization (also called "direction of easy magnetization"). In magneto-optical storages, one of the modes for writing data consists in utilizing magnetic transducers usually composed of a magnetic circuit around which a winding is arranged and which contains a gap. The variation of the induction within the gap permits the writing of data present on the medium associated with this transducer.
Thus, it is seen that the device for recording data on the magnetic disks of magneto-optical storages is most frequently formed by associating a magnetic recording layer of the magnetic disk with a magnetic transducer arranged opposite the latter, its gap being spaced a very small distance from the layer which ranges, say, between zero and several tenths of a micron.
In this type of devices for writing data with a very high surface density, magnetic integrated transducers are preferred. A description of such magnetic data carriers and a magnetic integrated transducer is found, for example, in U.S. Pat. No. 4,287,544.
Such magnetic integrated transducers generally comprise two pole pieces made of thin magnetic layers arranged on the same side of the data medium and having a gap in the vicinity thereof. The pole pieces enclose an electric winding formed of thin conducting superimposed layers which are separated from one another by thin insulating layers. The gap of the magnetic integrated transducer is placed such that its length is perpendicular to the direction of motion of the magnetic medium. The transducer moves past the base material at right angles to the plane of the two magnetic layers constituting the pole tips. During this motion, any magnetic domain of a track of the base material opposite which the transducer is placed moves, with time, succesively past a first pole tip called the "upstream pole tip" and then past the second pole tip called the "downstream pole tip". Preferably, if their dimension measured parallel to the direction of motion is called "pole tip thickness", the thickness of the downstream pole tip is substantially smaller than that of the upstream pole tip.
The writing of data on the medium is effected by causing the base material to move at a given speed and causing a variable current representing the data to be written to pass through the winding. This current causes the generation of a magnetic flux in the pole tips, the path of which is closed through the magnetic layer of the data medium. Opposite the downstream pole tip the magnetic flux is concentrated because of the thinness thereof. Since the axis of easy magnetization of the layer of data is at right angles to the surface of the magnetic recording layer, the magnetic field opposite the downstream pole tip is canalized, i.e., concentrated, in this direction. Opposite the upstream pole tip, the magnetic field is scattered and has a much smaller intensity than the magnetic field opposite the downstream pole tip. This permits non-saturation of the magnetic layer of data at the upstream pole tip, thereby allowing the downstream pole tip to write data in the best conditions. To sum up, it is obvious that in such prior art devices, for high density writing of data, only the downstream pole tip defines the nature of the data recorded on the base material, that is to say, especially the direction and the modulus of the magnetic induction in each of the domains created thereon. This is also true when the "downstream" and "upstream" pole tips have approximately the same thickness.
Thus, it is apparent that, although there are two pole tips (or, simply, two poles), only one pole tip writes data: in other words, there is only one writing pole, the second pole tip (or second pole) allowing only the flow of the magnetic flux generated by the winding of the transducer.
It is obvious that the two writing poles PEIl, PEI2 form the magnetic circuit of the transducer TWI which contains the gap GI.
In FIGS. 4 and 5, the transducer TMI and tte recording
The type of transducer for high density writing described above permits one to obtain longitudinal densities of the order of 2 to 4,000 changes per cm in the direction of magnetization and radial densities of the order of 4 to 500 tracks per cm. These densities are insufficient for the performances contemplated for magneto-optical storages.